The truth is much less surprising. Scientists working with the
intrepid robot have just confirmed that the rock (called Pinnacle
Island) was simply kicked up by one of Opportunity's wheels as it
made its way across the planet's surface. [ Amazing
Photos from NASA's Opportunity Mars Rover ]

"Once we moved Opportunity a short distance, after inspecting
Pinnacle Island, we could see directly uphill an overturned rock
that has the same unusual appearance," Opportunity deputy
principal investigator Ray Arvidson, of Washington University in
St. Louis, said in a statement. "We drove over it. We can see the
track. That's where Pinnacle Island came from."

The rock stirred up enough controversy that a concerned citizen
even filed a lawsuit
against the space agency, claiming that NASA failed to
properly investigate a possible fungus growing on the Red Planet.

Although researchers figured out where the rock came from, there
are other weird aspects of the Pinnacle Island tale. Using
Opportunity 's
tools, mission scientists have discovered that the rock has very
high levels of sulfur and manganese. Both of those elements are
water-soluble, suggesting that they were concentrated in the rock
due to the "action of water," NASA officials said.

"This may have happened just beneath the surface relatively
recently, or it may have happened deeper below ground longer ago
and then, by serendipity, erosion stripped away material above it
and made it accessible to our wheels," Arvidson said.

The rock is located in a spot on "Murray Ridge" along the wall of
Endeavour Crater, where Opportunity is spending the Martian
winter. Now that the rover is done examining Pinnacle Island, the
Opportunity team is planning to drive the rover uphill to check
out exposed rock layers on a different part of the Martian
surface.

"We are now past the minimum solar-energy point of this Martian
winter," Opportunity project manager John Callas, of NASA's Jet
Propulsion Laboratory in Pasadena, Calif., said in a statement.
"We now can expect to have more energy available each week.
What's more, recent winds removed some dust from the rover's
solar array. So we have higher performance from the array than
the previous two winters."

Opportunity has been exploring Mars since 2004, landing on the
Red Planet a few weeks after its twin, Spirit, touched down on
the Martian surface. Both rovers were assigned 90-day missions,
but Spirit gathered data until 2010 and Opportunity is still
roving along.